As to an antenna structure, i.e. the present dipole antenna, it is not easy to carry because of its size. For the present radio frequency identification (RFID), it could be easy to carry with users by providing a conventional meander line antenna (MLA), which is folded at least once for reducing its size.
Please refer to FIG. 1, which is a plan view showing a conventional meander line antenna. An inner-folding dipole antenna 1 includes a first dipole antenna 12 and a second dipole antenna 14. The first dipole antenna 12 and the second dipole antenna 14 are folded toward each other, respectively. Further, the first dipole antenna 12 and the second dipole antenna 14 are matched with a feeding point via a T-shaped network T. However, the length and the width of the inner-folding dipole antenna 1 are respectively 79 mm and 53 mm because of the limited dimensions for its half-wavelength.
Please refer to FIG. 2, which is a plan view showing a further conventional meander line antenna. A meander-line dipole antenna 2 includes a feeding portion 20 and a coupled loop 22 formed thereon. Further, there is a parasitic element 24 near to the coupled loop 22 for being coupled therewith. The parasitic element 24 is still made by the mentioned folding process. However, the length and the width of the meander-line dipole antenna 2 are respectively 80 mm and 17 mm because of its limited half-wavelength.
Please refer FIG. 3, which is a plan view showing a conventional dipole antenna. This is a radio frequency identification tag (RFID tag) 3 for Texas Instruments (TI). The RFID tag 3 includes a feeding portion 30 and a dipole antenna 32. Besides, there is a loop structure 34 near to the feeding portion 30 for increasing the matching inductance. However, the length and the width of the meander-line dipole antenna 2 are respectively 95 mm and 38 mm based on the limited factor for the half-wavelength.
According to above-mentioned description, the dimensions of all conventional antennas could be reduced by folding the antenna under a fixed bandwidth. Moreover, the dimensions of the antenna are limited by wavelength and frequency, and the available frequency is subject to the environment of the antenna. The bandwidth of the mentioned conventional antennas is between 70 MHz and 100 MHz. Accordingly, it is common to use the process for re-folding the antenna if the dimensions thereof would be further reduced. However, the characteristics of the antenna would be changed, and a quality factor (Q factor) is further reduced and the bandwidth thereof is further narrowed. Thus, such process is not available. Therefore, it is an important issue applied in this field of the antenna to reduce the dimensions of the antenna and maintain an original bandwidth thereof.
Therefore, the purpose of the present invention is to develop an antenna structure and a method for increasing its bandwidth to deal with the above situations encountered in the prior art.